A simple digital readout circuit for differential resistive or capacitive sensors

This paper introduces and analyzes a novel direct interface circuit (DIC) that directly connects differential resistive and capacitive sensors to digital processors (DPs), performing a magnitude-to-time-to-digital conversion of the information they provide. The simple circuit performs the readout using two passive components, the differential sensor and the DP. In some cases, the circuit may require an additional passive element. The DP only uses common digital resources such as bidirectional pins or a counter, meaning microcontrollers, FPGAs, or ASICs could all be used as DPs. Different DICs proposed in the literature for reading differential sensors require three time measurement processes to estimate the variable to be measured. The new circuit requires only one, saving time and energy dissipation and reducing the number of error sources. A design based on an FPGA has been implemented as a proof of concept. Measurement times in the order of 1.1–1.3 ms have been obtained with this configuration. Errors in the readout of a differential resistive sensor are below 0.34% in the worst case and below 0.63% for a differential capacitive sensor.